Devices for detecting material parameters in flexible and planar components in stressed or distorted states are known from the prior art. Material parameters are parameters that provide information on the linear-elastic deformation of a component, inter alia, e.g. the shear modulus as a material constant or the modulus of elasticity as a relationship between stress and distortion when a solid body is deformed and linear elastic behavior. To determine various material parameters, normal and shear stresses can be applied to planar samples. The material parameters can be calculated from the measured data using known material laws. According to the orthotropic material law, the material parameters are independent of one another, the moduli of elasticity and the transverse strain in the direction of orthotropy being independent of the shear modulus for an even stress state and an even distortion state. Previously, determining the material parameters required three differently designed tests, the determination of the transverse strain being limited to the linear material behavior. Therefore, normal stresses always have to be superposed to calculate the shear modulus.
For flexible materials, shear tests for determining the shear modulus require prestress in the direction of orthotropy to prevent the formation of creases in the material sample upon shear loading. Generally, what are known as shear frames are used, in which the sample is mounted in a prestressed manner and the square shear frame is pulled via the diagonals. The amount of prestress determines the deformation behavior, i.e. the greater the prestress, the smaller the shear distortions. The displacements of the sample can be measured by means of travel sensors or optical measurement methods.
Using this method, the material parameters can only be determined for linear stress-strain behavior. As soon as the behavior stops being linear, the material parameters can no longer be determined, or can only be determined with very low precision.
DE 10 2009 020 519 A1 describes a device for testing various parameters of high-strength flexible textile laminate materials. Biaxial loads are applied to a sample, with a view to measuring a force at which the material fails.
EP 2 570 791 A1 discloses a device for determining the biaxial strain parameters of a sample. The sample is cross-shaped and stress is applied to each of the four arms of the sample. A frame is constructed from frame parts that can move relative to one another and in which a fixed and a movable member are both associated with a tensile test machine.
U.S. Pat. No. 7,204,160 B1 describes a device by which any stress state can be applied to a cross-shaped, planar sample by means of a cylindrical test apparatus. In flexible samples having high transverse strain, the surface of the sample is bent twice. Since the influence of the double bend of the surface on the material parameters is unknown, the additional bend components cannot be calculated therefrom.
WO 2012/100 780 A1 also discloses an S-shaped biaxial measuring device, constructed from two fork supports for applying tensile stress, and two curved arms. The curved arms have a plurality of sample mounts that hold a sample arranged in the center.
To determine the transverse strain, the stress-strain behavior has to be linear. However, for non-linear material behavior, as is the case in particular with large distortions, i.e. of greater than 10%, there is no basis for separating the geometric non-linear components from the elastic components, especially in the case of a shear load.
The orthotropic material law for loads in the direction of orthotropy is based on the normal distortions being independent of the shear distortions. In flexible materials, crease-free shear loading is only possible if a normal stress state is superposed thereon. For composite materials having shear resistance, the moduli of elasticity have to be determined by breaking down the stress components in the filament direction when the load is not parallel thereto, i.e. the load is not applied in a (main) fiber direction of the composite material. For materials having low shear resistance, a non-parallel load of this kind leads to internal twisting, and known material laws, in particular for composite materials, are only applicable to a limited extent. The material parameters can then only be determined to an insufficient extent or for small distortions (in which the behavior is still linear).